1. Field of the Invention
The present invention relates to a fluoride base flux used for brazing aluminum members comprising aluminum or aluminum alloys.
2. Prior Art
In recent years, aluminum equipment used for heat exchangers, such as radiators and oil coolers of automobiles, condensers and evaporators of air conditioners and the like are generally fabricated by a brazing process. It is known that fluxes of chlorides or fluorides are usually used for brazing these aluminum members.
These fluxes are required to have such characteristics as having a lower melting point than aluminum members and brazing materials, having a cleaning effect for eliminating oxides or contaminants on the surfaces of the aluminum members, and having no corrosive properties against the aluminum members after brazing.
However, a drawback of chloride fluxes is that they need to be washed out with water after brazing since they are corrosive against the aluminum members or brazing materials.
On the other hand, fluoride fluxes do not have any corrosive properties against the aluminum members, and therefore, they do not need to be washed out. Thus, in recent years, the fluoride fluxes have been increasingly used for brazing aluminum members.
As fluxes of fluorides currently used for brazing aluminum members in a furnace of non-oxidative atmosphere, the following patents have been disclosed and known:
1. U.S. Pat. No. 3,951,328: A flux comprising potassium fluoroaluminate such as KAlF.sub.4 and K.sub.3 AlF.sub.6 with an AlF3/KF ratio of about 65:35 to 45:55 and having a melting point of about 560.degree. C. or higher; PA1 2. U.S. Pat. No. 4,579,605: A flux comprising 5 to 95 wt % of K.sub.2 AlF.sub.5 or K.sub.2 AlF.sub.5.H.sub.2 O and the remainder of KalF.sub.4 ; and PA1 3. U.S. Pat. No. 5,450,666: A flux containing at least 96 wt % of KAlF.sub.4 ; and the like.
However, as can be understood from FIG. 1, these fluxes have a minimum melting point of 560.degree. C.; and, when composed of KAlF.sub.4 only, the melting point is 574.degree. C.
With the recent years' requirement for reducing the brazing cost, there has been a demand for developing a flux that has a lower melting point so as to save brazing energy.
In order to accomplish this object, the addition of cesium fluoride (CsF) or lithium fluoride (LiF) to the flux system, or the use of a system comprising these compounds and aluminum fluoride (AlF.sub.3) have been proposed (for example, U.S. Pat. No. 4,556,165 and U.S. Pat. No. 4,689,092).
However, because cesium (Cs) and lithium (Li) are very expensive elements, these methods do not reduce the brazing cost, which is a disadvantage, even if they are able to lower the melting point.